PNG export in LibreOffice Calc

Both LibreOffice Writer and Impress has the ability to export the document as
PNG, which is one way to create thumbnails for documents — i.e. being able to
preview them before the real loading of the document happens. It turns out
Calc did not have this feature, and given that ScModelObj also supports the
css::view::XRenderable interface (just like Writer), I hoped that it won’t
be too complex to add one.